One type of non-volatile memory is resistive memory. Resistive memory utilizes the resistance value of a memory element to store one or more bits of data. For example, a memory element programmed to have a high resistance value may represent a logic “1” data bit value, and a memory element programmed to have a low resistance value may represent a logic “0” data bit value. The resistance value of the memory element is switched electrically by applying a voltage or a current to the memory element. One type of resistive memory is magnetic random access memory (MRAM). Another type of resistive memory is phase-change memory. While this invention is described with respect to phase-change memory, the invention is applicable to any suitable type of resistive memory.
Phase-change memory uses a phase-change material for the resistive memory element. Phase-change materials exhibit at least two different states. The states of phase-change material may be referenced to as amorphous and crystalline states. The states may be distinguished because the amorphous state generally exhibits higher resistivity than does the crystalline state. Generally, the amorphous state involves a more disordered atomic structure, while the crystalline state features a more ordered lattice. Some phase-change materials exhibit two crystalline states, e.g. a face-centered cubic (FCC) state and a hexagonal closest packing (HCP) state. These two crystalline states have different resistivities.
Phase change in the phase-change materials may be induced reversibly. In this way, the phase-change material may change from the amorphous state to the crystalline state, and from the crystalline state to the amorphous state, in response to temperature changes. The temperature changes to the phase-change material may be achieved in a variety of ways. For example, a laser can be directed to the phase-change material, electrical current may be driven through the phase-change material, or electrical current can be fed through a resistive heater adjacent the phase-change material. With any of these methods, controllable heating of the phase-change material causes controllable phase change within the phase-change material.
Thermal crosstalk occurs when heat generated within a phase-change memory cell or other resistive memory cell during a write operation of the memory cell is thermally conducted to a neighboring memory cell. During a write operation, there may be a large amount of heating within the selected memory cell, but neighboring memory cells should see no significant temperature rise. If the temperature rise at the location of the neighboring memory cell caused by the conducted heat is large enough, the state of the neighboring memory cell may be affected and the data stored therein may be corrupted.
Typical phase-change memories operating at room temperature are usually not affected by thermal cross-talk. For example, for a typical phase-change memory using Ge2Sb2Te5 for the resistive elements, the temperature increase of a neighboring phase-change memory cell during a reset operation is typically up to about 50° C. Therefore, this phase-change memory operating at room temperature typically has a maximum temperature below 110° C., which is the maximum temperature for an amorphous bit to withstand crystallization for more than 10 years. Therefore, this maximum temperature limits the phase-change memory data retention to 10 years. If, however, the phase-change memory is operating at an elevated temperature, such as 70° C., the intrinsic heat diffusion is no longer sufficient to guarantee that the neighboring phase-change memory cell temperature will remain below the 110° C. specified for 10 year data retention.